Composed of 4 nucleotides, that always pair the same. The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T C pairs with G Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. C G T A

Proteins carry out the process of replication. DNA serves only as a template. Enzymes and other proteins do the actual work of replication. Enzymes unzip the double helix. Free nucleotides bond to the strands as A with T and G with C. nucleotide The DNA molecule unzips in both directions.

DNA polymerase enzymes bond the nucleotides together to form the double helix. Polymerase enzymes form covalent bonds between nucleotides in the new strand. Two new molecules of DNA are formed, each with an original strand and a newly formed strand. DNA polymerase new strand nucleotide

RNA carries DNA’s instructions. Transcription converts one strand of DNA to RNA. The central dogma states that information flows in one direction from DNA to RNA to proteins.

The central dogma includes three processes. Replication Transcription Translation replication transcription translation RNA is a link between DNA and proteins.

RNA differs from DNA because RNA has uracil instead of thymine. There is no T. A pairs with U instead. Transcription Translation

Transcription is catalyzed by RNA polymerase. RNA polymerase and other proteins form a transcription complex. The transcription complex recognizes the start of a gene and unwinds a segment of it. start site nucleotides transcription complex

RNA polymerase moves along the DNA Nucleotides pair with one strand of the DNA. RNA polymerase bonds the nucleotides together. The DNA helix winds again as the gene is transcribed. DNA RNA polymerase moves along the DNA

The RNA strand detaches from the DNA once the gene is transcribed.

Translation converts mRNA into a protein (or polypeptide). Amino acids are coded by mRNA base sequences. A codon is a sequence of three nucleotides that codes for an amino acid. codon for methionine (Met) leucine (Leu)

The genetic code matches each codon to its amino acid or function. The genetic code matches each RNA codon with its amino acid or function. three stop codons Let’s look at CAU

A change in the order in which codons are read changes the resulting protein. Codons always code for the same amino acid. Amino acids join together to become a protein.

Summary Proteins